1. Field of the Invention
This invention relates to a solid-state image pickup device to be used for an image input apparatus such as a digital camera, a video camera, an image scanner or an AF sensor and also to a method of resetting the same.
2. Related Background Art
Typical examples of solid-state image pickup device include the CCD image sensor and the non-CCD image sensor. The former comprises a photoelectric converter having photodiodes and a CCD shift register, whereas the latter comprises a photoelectric converter having photodiodes or photo-transistors and a scanning circuit having MOS transistors.
The APS (active pixel sensor) is a type of non-CCD image sensor comprising a photodiode and MOS transistors.
More specifically, an APS comprises combinations of a photodiode, a MOS switch and an amplifier for amplifying the signal from the photodiode, each combination being arranged in correspondence to a pixel, and provides a number of advantages including the capabilities of “XY addressing” and “realizing the sensor and the signal processing circuit in a single chip”. On the other hand, it is rather difficult for an APS to reduce the chip size that is a determinant of the dimensions of its optical system because of the large number of elements it has for each pixel. This is the reason why the market for solid-state image pickup devices has been dominated by conventional CCD image sensors to date.
However, in recent years, due to the technological development for miniaturizing MOS transistors and the strong demand for “realizing the sensor and the signal processing circuit in a single chip” and “reducing the power consumption rate of the image sensor for the purpose of power saving”, the APS has been attracting attention as it is also called a CMOS sensor.
FIG. 23 of the accompanying drawings schematically illustrates the pixel section of a known APS and its operation.
Referring to FIG. 23, photoelectric converter PD is a buried type photodiode similar to the one typically used in a CCD. With a buried type photodiode, the dark current that can be generated on the SiO2 surface can be suppressed by arranging a p+-layer containing an impurity to a high concentration level on the surface and the saturation charge of the photodiode can be raised by the junction capacitance generated between the n-layer of the storage portion and the p+-layer of the surface. It operates in a manner as described below. Firstly, diffusion region FD is reset to a reference voltage by applying an on-pulse to gate RST. Thereafter, an off-pulse is applied to the gate RST to bring the diffusion region into a floating state in order to start storing data. After a certain period of time, another on-pulse is applied to gate TX to read the charge of the optical signal stored in the photoelectric converter PD to floating diffusion region FD, which is the input terminal of the amplifier of the APS, by way of MOS transfer section TX. Then, the signal charge Qsig is converted into voltage Qsig/CFD by means of the capacitance CFD of the floating diffusion region FD and the signal is red by way of a source-follower circuit that uses the floating diffusion region FD as its input terminal.
When a reverse bias voltage is applied to a buried type photodiode, depletion layers extend from the PN junction at the interface with the surface p+-layer and from the PN junction at the interface with the P-type well PWL perpendicularly into the n-layer. At this time, the number of electrons in the n-layer of the photodiode is substantially equal to that of the neutral region located between the two depletion layers and it is reduced in proportion to the width of the depletion layers. The number of electrons of said neutral region when the reverse bias voltage is 0 volt corresponds to the saturation charge Qsat. As the both depletion layers extend to contact each other by the reverse bias voltage, the inside of the photodiode is completely depleted to make the neutral region non-existent. The reverse bias voltage for making the neutral region non-existent will be referred to as depletion voltage Vdp in the following description.
If a reverse bias voltage greater than the depletion voltage is applied, the electron concentration of the n-layer of the photodiode decreases as an exponential function of the reverse bias voltage.
When the n-layer of the photodiode of an APS having a configuration as described above is depleted, the electric charge generated by light is almost totally transferred to the floating diffusion region FD to reset the electron state in the photodiode. A mode of transferring almost all the electric charge of the photodiode is referred to as depletion transfer hereinafter.
FIG. 24 of the accompanying drawings shows the relationship between the saturation charge of a photodiode and the voltage of the floating diffusion region FD upon reading the saturation charge and also the relationship between the saturation charge of the photodiode and the depletion voltage Z. Then, the voltage VFDsat of the floating diffusion region FD is expressed by the formula below:VFDsat=Vres−Qsat/CFD where Vres is the reset voltage, Qsat is the saturation charge of the photodiode and CFD is the capacitance of the floating diffusion region.
Generally, the saturation charge of a photodiode has to be higher than the level of realizing a desired sensitivity, which may be A in FIG. 24. For realizing a depletion transfer, on the other hand, the requirement ofVFDsat>Vdp has to be met, provided that Vdp is the depletion voltage of the photodiode, which is indicated by B in FIG. 24. If VFDsat<Vdp, the reverse bias voltage of the photodiode will become the voltage of the floating diffusion region to leave a neutral region within the photodiode and hence the signal voltage will be read out in accordance with the capacitance division between the capacitance attributable to the both depletion layers and the capacitance of the floating diffusion region. At the same time, electrons will remain in the photodiode by an amount equal to the saturation charge Qsat after the reading operation so that they can give rise to a residual image and a noise.
As described above, in the known APS, the saturation charge Qsat of the photodiode is made to meet the requirement of A<Qsat<B so as to be found within the interval of C in FIG. 24.
However, the saturation charge Qsat or the depletion voltage Vdp is apt to be affected by variances of the manufacturing process. For instance, the depletion voltage can be shifted by 0.4 volt when the dose of ion implantation for forming the n-layer of the photodiode is varied by 10%.
Then, the net result is a low manufacturing yield.